1. Field of the Invention
The present invention relates to a loud speaker for high frequency sound that reproduces sound signals and to a diaphragm used for such a loud speaker as well as to a process for a diaphragm.
2. Discussion of the Related Art
In recent years, as music sources to be reproduced are digitized, a loud speaker with more excellent characteristics has been in demand by audio related industries as a sound output apparatus. As for the characteristics of a loud speaker, improvements in a conventional loud speaker such as higher output sound level, lower distortion and flatter frequency response are required. In particular, gaining a diaphragm of a loud speaker for reproducing high frequency sound (also referred to as a tweeter) which greatly affects the quality of sound and establishing a making process therefore have grown in importance. In a conventional loud speaker for reproducing high frequency sound, a dome shaped diaphragm utilizing a polymer film or a resin in a sheet form is used. Here, the dome shaped diaphragm is manufactured by heating and molding the polymer film, or the resin in a sheet form, in a metallic mold.
FIG. 1 is a cross section view, showing an example of a structure of a loud speaker using a polymer film according to a prior art, which shows the right half of the loud speaker from the central axis. As shown in this figure, a dome shaped diaphragm 1 is a diaphragm formed by heating and applying pressure to a polymer film or to a resin material in a sheet form, wherein a dome part 2, a dome central part 3, a voice coil junction part 4, surrounds 5a and a frame pasting part 5b are formed. Here, the surrounds 5a and the frame pasting part 5b are referred to as a peripheral part 5. The frame pasting part 5b is defined as the part which is adhered to the attachment surface of a frame 13. The surrounds 5a are defined as the part of the peripheral part 5 which elastically changes through the vibration of the diaphragm 1. Such surrounds are referred to as a plane edge. In addition, the dome central part 3 is defined as the top part of the dome part 2 while the voice coil junction part 4 is defined as the lower part of the dome part 2.
A voice coil bobbin 6 is a cylindrical member formed of an aluminum foil, of a thin high polymer foil, of a sheet of paper, or the like. The top edge thereof is bonded to the voice coil junction part 4 by means of adhesive 7. A voice coil 7, which generates an electromagnetic driving force, is wound around the lower part of the voice coil bobbin 6. A top plate 8 in a circular form is arranged inside of the voice coil bobbin 6 while a yoke 9 in a cup form is arranged outside of the voice coil bobbin 6. In addition, a magnet 10 is arranged between the bottom surface of the top plate 8 and the flat plane surface of the yoke 9. The top plate 8, the magnet 10 and the yoke 9 form a magnetic circuit 11. Then the gap between the external periphery side of the top plate 8 and the internal periphery side of the yoke 9 becomes an annular magnetic gap 12.
The peripheral part 5 is formed in an annular plate and is attached to the frame 13 with the frame pasting part 5b intervened. The voice coil 7 is arranged in the annular magnetic gap 12 and allows the voice coil bobbin 6 to vibrate in a pistonic motion when a driving current corresponding to the audio signal is supplied so as to cause an electromagnetic driving force in the direction parallel to the central axis of the voice coil bobbin 6. This pistonic motion is conveyed to the voice coil junction part 4 so as to allow the diaphragm 1 to vibrate in the direction of the central axis. In the case that the rigidity of the diaphragm 1 is large and the equivalent mass thereof is small, the dome part 2 vibrates integrally when the dome central part 3 is included. At this time, the surrounds 5a are elastically transformed. In this manner, the phase of the sound radiated from the diaphragm 1 becomes uniform so that the volume velocity becomes equal to the audio signal.
A conventional making process for such a dome shaped diaphragm is concretely described in the following. FIG. 2 is a cross section view showing the structure of the main components of the metallic mold used for the production of the dome shaped diaphragm. This type of dome shaped diaphragm is conventionally used as a diaphragm of a tweeter for reproducing a high frequency range of sound. Then, as for the material thereof, in general, a resin material 20 in a sheet form is used. The thickness of the sheet is, for example, 50 xcexcm. FIG. 3 is a cross section view showing the structure of a dome shaped diaphragm 22 in the case that it is manufactured by using a metallic mold 21 of FIG. 2. This diaphragm 22 is partially different from the one shown in FIG. 1 and has a dome part 23, dome central part 24, voice coil junction part 25, cone part 26, roll-surrounds 27 and frame pasting part 28. However, the making process for dome shaped diaphragm 1 of FIG. 1 and the making process for dome shaped diaphragm 22 of FIG. 3 are essentially the same.
The metallic mold 21 shown in FIG. 2 is formed of heat pressure metallic mold 29, which is a male metallic mold, and a heat pressure metallic mold 30, which is a female metallic mold. The molding sides of the heat mold assembly 29 and the heat mold assembly 30 have approximately the same form and heaters for heating, 29a and 30a, are built in to the respective metallic molds. Each metallic mold has a molding side for the dome part, a molding side for the voice coil junction part, a molding side for the cone part, a molding side for the roll-surrounds and a molding side for the frame pasting part. The heat mold assembly 29 is attached to a shank 29b so as to be able to shift between the pressure position and the release position relative to the heat mold assembly 30, which is stationary.
In order to manufacturer a diaphragm 22, a resin material 20 in a sheet form is positioned on the pressure surface of the heat mold assembly 30 and electricity is turned on to the heaters for heating 29a, 30a of the respective metallic molds so as to heat the respective metallic molds to a predetermined temperature. Then, by pressuring the heat mold assembly 29, which is a male mold, via the shank 29b, the pressure between the two metallic molds is maintained at a predetermined value. Thereby, the resin material 20 is softened and melted so as to be plastically transformed into the shape of the molding size of the metallic mold 21.
The diaphragm 22 gained in such a manner has a dome form as shown in FIG. 3 and the thickness thereof varies depending on location. The frame pasting part 28 and the middle part of the dome part 23 become 50 xcexcm, which is the thickness of the material before molding, while there is a tendency of thinning such that the dome central part 24 becomes 20 xcexcm, the voice coil junction part 25 becomes 35 xcexcm and the roll-surrounds 27 become 40 xcexcm.
In this type of dome shaped diaphragm, though it is ideally desirable to secure the same thickness throughout the entirety, in many cases the pressure between the heat mold assemblies 29, 30 does not spread uniformly throughout the entirety of the diaphragm. Therefore, the thickness varies depending on respective locations within diaphragm 22. In particular, the thickness of the middle part of the dome part 23 differs greatly from the thickness of the dome central part 24 and the voice coil junction part 25. This is because the resin material 20 receives pressure which varies locally in strength when it contacts convex surface parts of the heat mold assemblies 29, 30 at the time of press molding so that the stretched portion expands its area and the thickness of each location varies so as to have uneven values. In particular, the parts essentially require rigidity for high frequency sound reproduction or for distortion reduction, such as the dome central part 24 and the cone part 26, become thin while other parts are formed to be thick. Therefore, distortion increases due to partial resonance and the amplitude of the thin parts become greater than is necessary at the time of resonance. Therefore, there is a problem wherein the peak of the sound level characteristics or the distortion increase. In addition, since the lower part of the dome part 23 becomes thinner, the transmission of the force from the voice coil 7 becomes insufficient and, therefore, there are problems wherein the high frequency range reproduction characteristics are lowered and the input-output characteristic deteriorates.
The diaphragm for a tweeter is required to have a flat frequency characteristic in a range of comparatively high frequency to be reproduced, to be high in sound conversion efficiency, to have broad directional characteristics, and the like. Therefore, most diaphragms have small dimensions, are conventionally formed in a dome form, as shown in FIG. 1, by heating and applying pressure to the resin material 20 in a sheet form or are integrally formed with the cone part 26 in a short cone form around the dome part 23, as shown in FIG. 3. Then, the frame pasting part 28, which has a flat annular surface so as to be fixed to the peripheral part of the frame 13 of FIG. 1, is formed around the outer periphery of the cone part 26.
In particular, the part which becomes the voice coil junction part 25 is pressed to the convex surfaces of the heat mold assemblies 29, 30 so that this junction location becomes thinner and more fragile than the other parts. In such a case, the vibration transmitted from the voice coil 7 to the voice coil bobbin 6 becomes attenuated at the voice coil junction part 25 due to the compliance so as to cause a transmission loss. Therefore, the vibration of a desired mode cannot be sufficiently transmitted. Thus, a sound reproduction faithful to an inputted audio signal cannot be expected and, in addition, the voice coil junction part 25 becomes weakened so that this part is transformed in response to a small input. Furthermore, there is also a problem that a deformation occurs at the adhesion step of the voice coil junction part 25.
In this manner, according to a conventional press molding, the resin material 20 is partially stretched by receiving strong or weak pressure at the time of the molding of the resin material 20 in a sheet form so that the uniformity of thickness of the dome shaped diaphragm 1 or 22 cannot be maintained throughout the respective parts. Variation in thickness at the same part becomes greater for individual diaphragms. Hence, variation occurs in frequency characteristics. In addition, a sufficient thickness cannot be secured in part where rigidity is required. It is practically impossible to gain control so as to achieve a desired rigidity by controlling the thickness of the diaphragm.
In addition, when a thin resin material 20 is utilized in the case of a sheet molding, curvature or other transformations easily occur in the frame pasting parts 5b or 28, which presses and fixes the frame 13 and there is the defect that strong attachment to the frame 13 throughout the entire circumference cannot be realized. Accordingly, in the case that large scale production and high quality of the diaphragm are secured, it is difficult to reduce the thickness of the resin material 20 to a certain level, or below so that the thickness is practically limited. In addition, since the resin material 20 is conventionally produced in equipment for mass production, problems arise such that material costs become high and the diaphragm cannot be manufactured at low cost in the case that the thickness does not meet the industrial standards or wherein the resin material is changed. Furthermore, the parts that become the frame pasting parts, 5b or 28, are processed so as to be punched out with the required outer diameter, using a press, and, therefore, there is the defect that the remaining part that is not punched out becomes waste so that material loss is increased.
A loud speaker of the present invention is provided with a diaphragm which has, at least, a dome part and a peripheral part wherein the thicknesses are set at design values in respective locations and which causes an air vibration, a voice coil bobbin in a cylindrical form which is connected to the diaphragm, a voice coil wound around the outer peripheral part of the voice coil bobbin and a magnetic circuit for providing an electromagnetic driving force to the voice coil. In such a structure, the amplitude of the diaphragm at the time of resonance in the high frequency region is controlled, the peak and dip of the high frequency region reproduction frequency is made to be minimal and the characteristic of low distortion in a broad frequency range is implemented.
In addition, a diaphragm of the present invention is characterized in that, by injecting material for molding from a gate by using a male mold assembly and a female mold assembly, a dome part in a substantially hemispherical form, a cone part that is positioned in an outer peripheral part of the dome part and that has a cone surface, surrounds or roll-surrounds that are positioned in an outer peripheral part of the cone part and that elastically support the cone part and a frame pasting part for being fixed to a loud speaker frame that is positioned in an outer peripheral part in the surrounds or in the roll-surrounds are co-molded so as to, respectively, have desired thicknesses. By co-molding the material using such an injection mold method, the thicknesses of the dome part, the cone part, the surrounds and the frame pasting part of the diaphragm are controlled to predetermined values and, thereby, desired frequency characteristics can be gained.
In addition, a first making process for a diaphragm of the present invention uses a male mold assembly and a female mold assembly and injects a material for molding from a gate and, thereby, a dome part in a substantially hemispherical form, a cone part that is positioned in an outer peripheral part of the dome part and that has a cone surface, surrounds or roll-surrounds that are positioned in an outer peripheral part of the cone part and that elastically support the cone part and a frame pasting part for being fixed to a loud speaker frame that is positioned in an outer peripheral part in the surrounds or in the roll-surrounds are co-molded so as to, respectively, have desired thicknesses.
In addition, a second making process for a diaphragm of the present invention carries out a cutting operation on a block of a metal-based material and, thereby, a dome part in a substantially hemispherical form, a cone part that is positioned in an outer peripheral part of the dome part and that has a cone surface, surrounds or roll-surrounds that are positioned in an outer peripheral part of the cone part and that elastically support the cone part and a frame pasting part for being fixed to a loud speaker frame that is positioned in an outer peripheral part in the surrounds or in the roll-surrounds are integrally processed so as to, respectively, have desired thicknesses.